NEMA spec question - hazardous environments

[loicalset]

Hello,

I am looking to design a NEMA 8 enclosure and would like to know if anybody has experience with what exactly the spec means. I can't decipher if it means the electronics enclosure will be filled with oil or if it will be submerged in oil. Oil submersion makes more sense to me, but I haven't seen that in practice yet, so I don't know if that is something standard. Any tips?

The spec reads, "Type 8 enclosures and enclosed devices are arranged such that all arcing contacts, connections, and the like are immersed in oil. Arcing is confined under the oil such that it will not ignite an explosive mixture of the specified gases (hydrogen, cyclopropane, gasoline, acetlyene, etc) in internal spaces above the oil or in the atmosphere surrounding the enclosure. Enclosed heat generating devices shall not cause external surfaces t oreach temperatures capable of igniting explosive gas-air mixtures in the surrounding atmosphere. Type 8 enclosures:
a. Provide, by oil immersion, a degree of protection to a hazardous gas environment from operation of internal equipment.
...
c. Withstand a series of operation design tests with oil levels arbitrarily reduced and with flammable gas-air mixtures introduced above the oil.

That's pretty much it. Ideas?

 

[MacGyverS2000]

I do not know the answer, but if I were interpreting that paragraph, it reads as if the enclosure holds the oil.

The sentences "...all arcing contacts, connections, and the like are immersed in oil." and "...in the atmosphere surrounding the enclosure." tell me the explosive gas is surrounding the NEMA 8 enclosure, which encloses arcing contacts immersed in oil.

Of course, part c. muddies the waters a bit... "...with flammable gas-air mixtures introduced above the oil." This would tell me the enclosure itself might be immersed in oil.

Sounds like a poorly written spec (are there any other kind?).


Dan - Owner

http://www.Hi-TecDesigns.com

 

[loicalset]

Honestly! I can't believe they charge money for these things...

The rest of the spec does not give any insight. Maybe I should try to find NEMA 8 equipment and see how it is being used. Thanks for the input, mac.

 

[itsmoked]

I've never seen NEMA 8 in an application. Imagine the headaches!

Generally, when you see don't see something done commonly, it's because many others have looked at it, or tried it, and found it to be worse than alternatives.

I would review my need for such a brute force solution before trying much more to understand that confusing text. Bleeeech!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

It sounds like an old oil circuit breaker or compensator. They were used for switching power circuits and starting large motors. The contacts and resulting arcs were immersed in oil. This was not electronics but power circuits. Not suitable for use in hazardous environments because a severe fault could and would blow the oil out of the switch. (All over the switch room and the operator.)

But back to the spec;

Arcing is confined under the oil such that it will not ignite an explosive mixture of the specified gases (hydrogen, cyclopropane, gasoline, acetlyene, etc) in internal spaces above the oil or in the atmosphere surrounding the enclosure.

That seems clear and I don't see anything about immersing the enclosure in oil.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MacGyverS2000]

Bill, now that I reread it, it does appear a little more clear to me. "in internal spaces above the oil" says gases inside an enclosure not fully filled with oil. That would agree wiht my interpretation of the first part of the code.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[loicalset]

Wow, I guess we need to design a filling valve for the enclosure so that we can top it off with oil. Maybe a dipstick or something like that too...

The spec is from our customer and believe me, we would like drop that requirement, as I don't think an oil filled enclosure will play nice with RF data trying to transmit and receive. (the plot thickens...)

 

[LionelHutz]

Look at oil filled transformers with tap changers as possible examples of type 8 equipment. As Bill posted though, those can explode too.

The only electronics I've seen submerged in oil was computer hardware where the individual was trying to impress friends and others on the internet with their overclocking capabilities.

If you do this, you will likely have to qualify it with some type of testing or approval. I know I would not accept a box from some supplier that is to be installed in a hazardous location and claimed "NEMA 8" but not tested. The insurance company probably wouldn't like it to much either.

 

[rbalex]

Just a comment, while NEMA 250 implies Type 8 enclosures are suitable for all Class I locations, the NEC limits the "protection technique" to Division 2. See Section 500.7(I).